Check this code
Create one settings named MySettings with Type System.Collections.Specialized.StringCollection in settngs window
Save your settings
StringCollection myCol = new StringCollection();
String[] myArr = new String[] { "RED", "orange", "yellow", "RED", "green", "blue", "RED", "indigo", "violet", "RED" };
myCol.AddRange(myArr);
Properties.Settings.Default.MySettings = myCol;
Properties.Settings.Default.Save();
Getting it from settings
StringCollection myCol2 = new StringCollection();
myCol2 = Properties.Settings.Default.MySettings;
hope it helps